Answer:
Kittens can respond to catnip as early as 8 weeks old. The positive reaction and desire for catnip is inherited from their mother - if mum likes it then kittens will too.
All cats (in your case kittens) can, it is like a drug to them, but it is not like it hurts them like drugs hurt us.=D
Also keep in mind about 40% of the cat will not react to catnip.